In conventional reciprocating piston engines, ambient air is typically pulled inside an engine cylinder during an intake (or induction) stroke of a piston. The volumetric efficiency, which is the amount of air inducted into the engine cylinder by the piston divided by the cylinder volume, is limited both by the atmospheric pressure and the change in pressure needed to bring air into the cylinder. Increasing the volumetric efficiency reduces relative engine losses, increases engine efficiency, and also increase the power output of the engine without increasing a displacement of the engine. A related common trend is engine downsizing, which means a size of the engine is reduced in order to decrease engine losses significantly, while maintaining about the same amount of power output from the engine.
In order to improve the volumetric efficiency of naturally aspirated engines, two forced induction devices may be typically used; a turbocharger or a supercharger. A supercharger typically comprises a compressor in driving engagement with an engine crankshaft to compress additional air before intake into the engine. Superchargers will not be discussed in detail herein as they do not recuperate the kinetic energy from an exhaust gas flow; instead superchargers increase the power of the engine by increasing the volumetric efficiency of the engine.
FIG. 1 shows a cut-through sketch of a turbocharger 100 known in the prior art. A turbine 102, which is a radial inflow turbine expander, is shown which has an intake port 104 where the exhaust gas flow enters the turbine 102 radially and leaves through an outlet port 106 axially. A plurality of blades 108 of the turbine 102 allow for a recuperation of kinetic energy from the exhaust gas flow, which is directed to a central rotor hub 110. The central rotor hub 110 is also drivingly engaged with a compressor 112, in which a flow of air enters an intake port 114 axially and is pushed radially to an outlet port 116 by a plurality of blades 118 of the compressor 112. Due to inherent limitations in the design, the turbocharger 100 is subject to several issues that may be solved by using a complex control methodology or through the addition of costly technologies to the turbocharger 100.
One issue associated with such a turbocharger is a maximum boost pressure that the engine can withstand without damage to components of the engine due to increased pressure. Further, knocking of the engine may damage the turbocharger. A boost pressure increases depending on am amount of exhaust gases, as the compressor is directly linked to the turbine. At a certain point, pressure has to be limited to avoid engine knocking and other potential damage related to the increased pressure at an intake manifold of the engine. This issue is commonly corrected through the use of a wastegate. The wastegate diverts a portion of the exhaust gas from the turbine, thus limiting the pressure and amount of energy that can be recuperated by the turbine. In a conventional configuration of a turbocharger, the excessive wasted exhaust and the complex control of the wastegate cannot be avoided.
Another issue associated with such a turbocharger issue is a dynamic known as turbo lag. Turbo lag is a time required to adjust a power output of the turbocharger in response to an adjustment in a throttle of the vehicle. Turbo lag is caused by an amount of time needed to generate a required pressure boost by an exhaust system and the turbine. Turbo lag significantly depends on the inertia of the components of the turbocharger, an amount of friction within the turbocharger, and an initial speed of the turbocharger, and an amount of exhaust gas passing through the turbine. A number of ways exist to decrease the turbo lag. For example, it is possible to decrease the rotational inertia, to change the aspect ratio of the turbine, to use variable geometry components, amongst other improvement, but all improvements significantly affect a cost and complexity of the turbocharger.
Another issue associated with such a turbocharger is a boost threshold. Turbochargers start producing boost only when enough energy can be recuperated by the turbine. Without the required amount of kinetic energy, the turbocharger will not be able to provide the required amount of boost. An engine speed at which this limitation disappears is called a boost threshold speed. The boost threshold speed is dependent on an engine size and an operating speed of the engine, a throttle opening, and a design of the turbocharger. As a result of the boot threshold, an operator of a vehicle including the turbocharger may notice an ineffectiveness of the turbocharger when the engine is operated under a certain speed.
A final issue associated with such a turbocharger is based on an energy recuperation capability of the turbocharger. The turbine of the turbocharger is only able to recuperate energy from the exhaust gas flow to compress intake gases. If the operator of the vehicle requests a low amount of power output from the engine, compression of the intake gases is not necessary, and all of the kinetic energy in the exhaust gas flow is directed around the turbine using the wastegate. Directing the exhaust gas flow around the turbine using the wastegate is an inefficient manner of operation for the turbocharger.
It would be advantageous to develop a turbocharger for an internal combustion engine that is simply controlled, reduces turbo lag, decreases a boost threshold of the turbocharger, and increases an efficiency of the internal combustion engine.